Reference is made to copending U.S. patent application Ser. No. 555,864 filed 11-23-83 and Ser. No. 555,865 filed 11-23-83.
The present invention relates to a color television (TV) imaging apparatus with an image tube which is capable of generating composite color TV video signals by means of a color separation stripe filter. More particularly, the present invention is concerned with a color TV imaging apparatus which improves color reproducibility in a dark scene and produced a high quality picture with a good S/N ratio.
A modern color TV camera employs a color TV imaging device in which a color separation stripe filter is disposed in a photoelectric transducer section to generate composite color TV video signals. Typical of such color TV camera is a single tube type color TV camera which uses as its imaging device an image tube having a photoelectric transducer section with a color separation stripe filter, or single plate type color TV camera whose imaging device is a solid state image sensor having a photoelectric transducer section with a color separation stripe filter. Because such types of TV cameras are easy to produce in a simple, compact and light-weight construction, exclusive study has been conducted for better performance and maneuvability for public use. Indeed, various types of products with such capabilities are now in the market. In a color separation stripe filter of the type described, a plurality of color stripes are arranged in a predetermined repetition mode, at least one color of the stripes being a complementary color. Generally, a color TV camera comprises a color TV imaging device using the stripe filter to generate a composite color TV video signal, and at least two demodulators for producing color signal components by demodulating color signals which are contained in an output signal of the imaging device and processed into carriers as will be described. The imaging element includes a photoconductive layer in the photoelectric transducer section.
Examples of the color separation stripe filter applicable to a color TV camera are shown in FIGS. 1 and 2. In FIG. 1, the filter has therein stripes W transparent for all color light, stripes G transparent for green light, and stripes Cy transparent for cyan light. The stripes W, G and Cy are arranged in a predetermined repetition sequence. In FIG. 2, the filter has therein stripes W transparent for all color light, stripes Ye transparent for yellow light, and stripes Cy transparent for cyan light, the stripes being arranged in a predetermined repetition mode.
A composite color TV video signal output from the imaging device with the filter shown in FIG. 1 or 2 is applied to a first low pass filter to become a brightness signal having a predetermined wide frequency band, to a second low pass filter to become a brightness signal having a predetermined narrow frequency band, and to a band pass filter. From the band pass filter, the video signal is fed to a red and blue (RB) separator circuit which then produces two different signals which are to be demodulated.
The two signals mentioned above are individually demodulated by discrete demodulators in the imaging apparatus. The resulting red signal and blue signal are delivered to a matrix circuit together with the narrow band brightness signal output from the second low pass filter. As a result, signals representing three primary colors, red, blue and green, individually appear at output terminals of the matrix circuit. Further, detailed explanation of this type of color signal processing for a single tube color camera may be found in U.S. Pat. No. 3,846,579 by same applicant.
By the above procedure, the composite color TV video signal output from the imaging device is processed into three primary color signals. A problem encountered with this type of color TV camera is that color reproducibility is poor in a dark scene (dark area). In light of this, we have extended study and research to clear up the cause of deterioration in the color reproducibility in a dark scene and found that such deterioration does not occur when the signals to be demodulated are greater than the peak-to-peak value of noise contained in the signals. This is the principle which opened up the way for further improvements.
Now, in a composite color TV video signal output from an image tube with a color separation stripe filter, a high frequency component appears as if a carrier wave having a frequency corresponding to the repetition frequency of the stripes in the filter and subjected to balanced modulation by a color signal. This specific kind of signal will hereinafter be referred to as "carrier color signal" for convenience. Assume that the carrier color signal applied as an input signal to a demodulator is C and the noise appearing at the input of the demodulator is n, which are respectively expressed as: EQU C=Ec sin (.omega..sub.c t+.phi..sub.c) Eq. (1) EQU n=En sin (.omega..sub.n t+.phi..sub.n) Eq. (2)
Assuming that the signal applied to the demodulator is S, it is the sum of the signal C produced by the Eq. (1) and the noise n produced by the Eq. (2): EQU S=C+n Eq. (3)
Expansion of the Eq. (3) results in: ##EQU1## where EQU .DELTA..omega.t=.omega..sub.n t-.omega..sub.c t
Assuming that a signal produced by amplitude demodulation of the signal S shown above is Sd, it is expressed as follows: ##EQU2##
Closely examining the Eq. (4), the signal Sd may be produced as follows depending upon the condition:
when Ec&gt;&gt;En EQU Sd=Ec+En cos (.DELTA..omega.t+.phi..sub.n -.phi..sub.c) Eq. (5) PA0 when Ec&lt;&lt;En EQU Sd=En+Ec cos (.DELTA..omega.t+.phi..sub.n -.phi..sub.c) Eq. (6) PA0 when Ec=En EQU Sd=Ec{1+(En/Ec).sup.2 }.sup.1/2 {1+cos (.DELTA..omega.t+.phi..sub.n -.phi..sub.c)}.sup.1/2 Eq. ( 7) PA0 when Ec nearly equals En ##EQU3## where ##EQU4## A=.DELTA..omega.t+.phi..sub.n -.phi..sub.c.
It will be seen from the above that the amplitude-demodulated signal Sd is represented by the Eqs. (4)-(8) depending upon the amplitude relationship between Ec and En.
When the amplitude-demodulated signal Sd appears as a picture on a tube, the viewer integrates it with his eyes to recognize it as a signal Sdd which has a mean value of the signal Sd with respect to time. Therefore, the signal Sdd corresponds to the DC terms of the Eqs. (5)-(8) and is expressed as: ##EQU5##
The relationship between Ec and the signal Sdd represented by the Eqs. (9)-(12) is indicated by a solid curve in FIG. 4, in which the characteristic indicated by a dotted line is ideal one attainable when the noise En is zero. As will be understood from FIG. 4, a level offset corresponding to the noise is added to the signal Sdd in a dark area thereby masking the pure signal component.
When the carrier output signal of the image tube increases as the envelope thereof is shown in FIG. 3, the demodulated signal Sdd naturally follows the solid curve shown in FIG. 4. As clearly shown in FIG. 4, in the range where Ec&gt;1, Ec and Sdd are substantially linearly proportional to each other while in the range where Ec&lt;1, the original signal is hardly produced and the color reproducibility in dark areas is masked by noise.
In this manner, concerning a color signal produced by demodulation, the color reproducibility is significantly deteriorated in dark areas when the signal before demodulation becomes smaller than the peak-to-peak value of the noise contained therein. On the other hand, the signal such as Y.sub.L in FIG. 5 which is not subject to demodulation, is not deteriorated in dark areas so that as a result of matrixing with two other denodulated signals white balance will be upset in dark areas if no correction is made.